Description
The Incoming Leg Control Model (ILCM) is a core architectural concept within the 3GPP IP Multimedia Subsystem (IMS), defined primarily in TS 23.218. It describes the functional model and procedures for controlling the 'incoming leg' of a session—the signaling path from the network towards the called party. In IMS, a session (like a voice or video call) is conceptually split into two legs: the originating (or outgoing) leg from the caller to the network, and the terminating (or incoming) leg from the network to the callee. The ILCM governs the latter.
The model centers around the Serving-Call Session Control Function (S-CSCF), which acts as the central node for session control. When a session request targets a user, the I-CSCF (Interrogating-CSCF) locates the user's assigned S-CSCF. This S-CSCF then instantiates the ILCM for that session. The ILCM encompasses the S-CSCF's logic for processing incoming SIP requests (like INVITE), interacting with application servers (AS) for service logic execution, enforcing subscriber profiles, and ultimately routing the session towards the called party's User Equipment (UE) via the P-CSCF (Proxy-CSCF). It handles critical functions such as authentication, authorization, initial filter criteria (iFC) evaluation, and charging triggers.
Key components within the ILCM's operation include the initial Filter Criteria (iFC), which are a set of rules downloaded as part of the user's service profile from the Home Subscriber Server (HSS). The S-CSCF uses these iFCs to decide when and which Application Servers to involve in the session setup. The model also defines interactions with the Breakout Gateway Control Function (BGCF) for sessions breaking out to the PSTN and with the Media Resource Function (MRF) for media processing. The ILCM ensures a standardized, service-agnostic control plane for delivering multimedia services, enabling features like call forwarding, voicemail, and number translation in a consistent manner.
Purpose & Motivation
The ILCM was created to provide a standardized, robust, and scalable model for controlling the termination side of multimedia sessions within the IMS architecture. Prior to IMS, traditional circuit-switched telephony had well-defined but rigid call models. The shift to all-IP networks required a new, flexible control model that could support a wide range of services beyond simple voice calls. The ILCM, along with its counterpart the OLCM (Originating Leg Control Model), provides this by separating call control logic from the underlying transport, enabling rapid service innovation.
Its introduction in 3GPP Release 5 (the first IMS release) addressed the need for a common service delivery platform. The ILCM solves the problem of how to uniformly apply subscriber-specific services (like call screening, forking, or pre-paid charging) to incoming session requests, regardless of the caller's location or network. By centralizing control in the S-CSCF and using a profile-driven filter mechanism (iFC), it allows operators to deploy new services on application servers without modifying the core session control logic every time.
Furthermore, the ILCM enables network interoperability and roaming. It defines a clear demarcation point (the S-CSCF in the home network) where the home operator's service policy is applied, even when the user is roaming. This model has been foundational for the global deployment of VoLTE, VoNR, and Rich Communication Services (RCS), ensuring consistent behavior and feature delivery across different networks and device implementations.
Key Features
- Defines control procedures for the terminating (callee) side of an IMS session
- Centrally implemented in the Serving-Call Session Control Function (S-CSCF)
- Utilizes Initial Filter Criteria (iFC) for trigger-based service invocation
- Manages interactions with Application Servers (AS) for service logic
- Handles routing decisions towards the called party's UE or breakout to PSTN
- Supports essential functions like authorization, charging, and media policy control
Evolution Across Releases
The Incoming Leg Control Model (ILCM) was first introduced as part of the foundational IP Multimedia Subsystem (IMS) architecture in Release 5 (often referenced from R99 groundwork). It established the core model where the S-CSCF controls the incoming leg using initial filter criteria to invoke services, defining the basic procedures for session termination control.
Defining Specifications
| Specification | Title |
|---|---|
| TS 23.218 | 3GPP TS 23.218 |